Leptospirosis is a zoonotic disease of worldwide distribution that does not lend itself to eradication--only control. Man and animals become infected primarily through contact with water or soil contaminated with urine from animals in the shedder state. The mechanism of leptospiral pathogenicity has not been established, and the efficacy of treatment of this disease with antibiotics, once it is symptomatic, is questionable. Classification of leptospires is still based on normal habitat and antigenic composition. Parasitic (virulent and avirulent) and saprophytic leptospires will be compared as to nutritional, metabolic, immunologic, and morphologic characteristics. Survival studies of parasitic leptospires in water and soil will be made. This investigation should provide improved methods for the isolation and identification of leptospires and characteristics associated with virulence which will aid in our understanding of the mechanism of pathogenicity and improve the treatment of the disease. Survival studies should permit an assessment of the potential of soil and water for the transmission of leptospirosis.